In U.S. Pat. No. 3,559,406, incorporated herein by reference, I disclosed a servomotor for a braking system having concentric pistons located in a pressurizing chamber for sequentially producing first and second operational braking forces. A pressure differential responsive wall in the servomotor moves the concentric pistons simultaneously until an optimum pressure differential develops within the servomotor. Thereafter, a manual input applied to the concentric pistons provides an additional pressurization force for moving the pistons in the pressurizing chamber. Unfortunately, the additional pressurizing forces acts on all the concentric pistons thereby reducing the effect of the manual input.
Later, in U.S. Pat. No. 3,910,048, I disclosed a two stage servomotor having a locking means for holding an exterior concentric piston in a stationary position during a transfer of a manual operational input to an interior piston. A valve in the exterior piston allows fluid to flow to the back side of the exterior piston during the simultaneous movement of the concentric pistons. The manual input from the operator moves the interior piston away from the exterior piston and allows a spring to close a flow-through valve. With the flow-through valve closed, the fluid on the back side of the exterior piston resists expansion of the pressurizing chamber thereby essentially eliminating any loss of pressurization through the manual input.